In recent years, wire saws have been known as means for simultaneously cutting a large number of thin sections (for example, wafers) from a workpiece such as a semiconductor ingot. Such a wire saw comprises a wire-feeding bobbin, a plurality of guide rollers, and a wire-winding bobbin, wherein a cutting wire extracted from the wire-feeding bobbin is wound around each of the guide rollers and then retrieved by the wire-winding bobbin. The cutting wire is driven at high speed in a longitudinal direction of the wire by respective rotations of both bobbins and the guide rollers. While the wires driven, a workpiece including the semiconductor ingot or the like is cutting-fed in a direction perpendicular to a plurality of wires aligned between specific guide rollers, and thereby a large number of thin sections are simultaneously cut from the workpiece.
With such a wire saw, it is required to apply tension equal to or greater than a certain level to the wire for cutting a workpiece in a stable manner; however, the greater the tension, the greater a mechanical load acting on the wire-winding bobbin. In other words, the greater the tension in a wire wound by the wire-winding bobbin, the greater the likelihood of the wire biting into an outer peripheral surface of the wire-winding bobbin, resulting in an increased risk in shortening life of the bobbin. Besides, in the case of the wire on which fixed abrasive grains are bonded, there comes an inconvenience in that the abrasive grains are likely to fall off because of an increase in a force of bringing the wires into pressured contact with each other on the wire-winding bobbin.
As means to solve such an inconvenience, it may be considered to use a tension adjuster to reduce winder-side wire tension to below feeding-side wire tension. However, the wire saw described above often adopts, as a system for driving the wire, a reciprocal drive system of running the wire alternately in a forward direction (a direction from the wire-feeding bobbin to the wire-winding bobbin) and a backward direction (a direction from the wire-winding bobbin to the wire-feeding bobbin); if a wire saw with such a reciprocal driving system is applied with the above performing tension adjustment, there may occur a significant decline in processing efficiency.
Specifically, according to the reciprocal driving system, since a sudden reversal of a wire driving direction causes a strong impact, there is required, in reality, a manipulation which involves first decelerating to stop the wire, then restarting driving in a reverse direction from a stopped state, and gradually accelerating the wire up to a target speed. Such a reversal of driving directions can be time consuming. In addition, for reversing the direction of driving of the wire, tension in the wire on a winding side which had been reduced must be increased and, conversely, tension in the wire on a feeding side which had been given high tension must be reduced. Moreover, if such a fluctuation in tension is performed in a sudden manner, there occurs a possibility of a break in the wire or the like; therefore, increasing and decreasing the wire tension also has to be performed in a gradual manner over a certain amount of time. Hence, in addition to the reversal of the driving direction of the wire, an increase or decrease in tension accompanying such a reversal also consumes time.
In consideration therewith, Japanese Patent Application Laid-open No. 2007-276054 below proposes performing an adjustment to increase tension in the wire on a winding side which had been reduced and to reduce tension in the wire on a feeding side which had been given high tension, during deceleration of a wire driving speed for reversing the driving direction of the wire, thereby reducing time loss.
However, with this method, since the adjustment of increasing wire tension on the winding side and reducing wire tension on the feeding side is performed during deceleration of a wire driving speed, there is a risk that a wire portion where tension is reduced by the adjustment may enter a region between guide rollers, that is, a region where a workpiece is cut, which may adversely affect processing accuracy of the workpiece.